Embodiments of the present invention relate to a wind turbine blade and a method of moulding a wind turbine blade tip section.
There is a trend among offshore wind turbine blades for increasing the length of the blade. This is because the power available from a wind turbine blade increases with the square of the radius of the blade. Thus, increasing the radius of the blade produces an increase in power output which is disproportionate to the cost of turbine itself and can contribute to a lower cost of energy.
However, the increase in blade size creates a number of technical challenges.
One of these relates to the tip section of the aerodynamic fairing.
Wind turbine blades have generally been made with fairings which extend for the full length of the wind turbine blade. These are generally made in two half shells which are bonded together on either side of an elongate spar in order to form the completed blade. In order to be able to transport longer blades one proposal is to make the blade in a traditional manner and cut it in half before reassembling it closer to the site of the wind turbine blade. This is a cumbersome process requiring ever larger joints between the cut sections as the blade length increases.
One successful attempt to overcome this problem is to make the blade of a number of modular sections for the spar and fairing which can be assembled closer to the site of the wind turbine. This does not require any blade sections to be cut up and re-joined. It is therefore much more readily scalable.
An example of this is disclosed in our own earlier WO 2009/130467 which has a central modular spar with a number of aerodynamic fairing sections arranged along the windward side and a similar number arranged along the leeward side. These are then connected to the spar and are attached to one another along their radially extending edges. These joints are deliberately offset from being directly at the leading and trailing edges of the aerodynamic fairing to ensure that the joints are kept away from the leading and trailing edges thereby ensuring accurate geometry at both the leading and trailing edges and protecting the joints from wear.
The mass of the blade in the tip region has a disproportionate effect on the blade in terms of the bending moments that it induces on the remainder of the blade. Therefore, it is desirable to have a lightweight tip that reduces loads on the blades and the rest of the turbine.
As the tip is moving faster, any imperfections in the shape of the fairing have a disproportionate effect on the aerodynamic efficiency of the blade and cause increased noise problems. Even greater care is therefore required in the making of the joint in this region.
Further, the longer the blade, the more prone that it is to a lightning strike. While blade tips are provided with openings to allow moisture from condensation to escape, these can often be blocked such that there is a significant amount of moisture which is trapped in the blade tip by the centrifugal forces. When struck by lightning, this liquid vaporises causing a rapid pressure increase in the tip section. This pressure increase can force the tip section apart at the joints between the windward and leeward sides.